Digital Speckle Photography Research Articles

Measurement of out-of-plane deformation by combination of speckle photography and speckle shearing interferometry

Summary We measure the derivative of out-of-plane deformation in presence of large in-plane rigid body displacement (RBD) by a combination of Electronic Speckle-Shearing Pattern Interferometry (ESSPI) and speckle photography. We use digital speckle photography in order to compensate for the in-plane RBD and eliminate any decorrelation effects. ESSPI techniques are then applied to the compensated speckle images to obtain the derivative of the out-of-plane deformation. The method is tested on an aluminum plate subjected to simultaneous out-of-plane deformation and large in-plane RBD. The results indicate that the proposed method preserves the fringe contrast observed in ESSPI.

Three-dimensional deformation field measurement with digital speckle correlation.

Digital speckle correlation is based on a detailed analysis of changes in speckle images that are recorded from laser-illuminated rough surfaces. The two in-plane components are obtained by cross-correlation of corresponding subimages, a method also known as digital speckle photography. The local gradient of the hitherto inaccessible out-of-plane component is determined from the characteristic dependence of the speckle correlation on the spatial frequency. A detailed experimental study is carried out to analyze the new technique for systematic and random measuring errors. For moderate decorrelation the accuracy of the out-of-plane measurement is better than lambda/10 and thus comparable with interferometric techniques. Yet the extremely simple and robust optical setup is suited to nondestructive-testing applications in harsh environments. The quality of the deformation maps is demonstrated in a practical application.

Quasi-real time bio—Tissues monitoring using dynamic laser speckle photography

Joint development of a laser monitor for the real-time bio-tissue analysis is presented. The monitor is based on the digital dynamic laser speckle photography and deals with soft and hard bio-tissues. In soft tissues, the dynamic bio-speckles are formed in a scattered from a tissue laser light. An optically transparent model of hard bio-tissue was prepared and preliminary analysis of a stress field in the stressed model was performed using the dependence of the refractive index of transparent solids upon the state of stress and the double exposure speckle photography data. The refractive index of the stressed material was evaluated and the state of stress was reconstructed using the stress-optical law.

New Methods of Laser Anemometry in Investigations of Combined Gasdynamic Flows

New developments in laser anemometry which are based on digital technologies of input and processing of images using CCD chambers and personal computers are analyzed. The capabilities of digital dynamic speckle photography are illustrated by the intensity fields obtained for surface blood flow in biotissues. Two‐*;phase flows are investigated with the use of a new PIV technique. New developments in the field of laser anemometry which are based on the use of coherent optical Doppler processors and enable us to pass from the local diagnostics to the diagnostics of two‐dimensional fields of velocity and its visualization are described.

Application of digital speckle photography to measure thickness variations in the vacuum infusion process

AbstractA new method to measure the movement of the flexible bag used in vacuum infusion is presented. The method is based on an in‐house developed stereoscopic digital speckle photography system (DSP). The advantage with this optical method, which is based on cross‐correlation, is that the deflection of a large area can be continuously measured with a great accuracy (down to 10 μm). The method is at this stage most suited for research but can in the long run also be adopted in production control and optimization. By use of the method it was confirmed that a ditch is formed at the resin flow front and that there can be a considerable and seemingly perpetual compaction after complete filling. The existence of the ditch demonstrates that the stiffness of the reinforcement can be considerably reduced when it is wetted. Hence, the maximum fiber volume fraction can be larger than predicted from dry measurements of preform elasticity. It is likely that the overall thickness reduction after complete filling emanates from lubrication of the fibers combined with an outflow of the resin. Besides, the cross‐linking starts and the polymer shrinks. Hence, the alteration in height will continue until complete cross‐linking is reached.

Microscopic 3-D displacement field measurements using digital speckle photography

A technique to measure object shape and 3-D displacement fields in micro-scale is offered by microscopic stereo digital speckle photography. The displacement of the random features that are often present on many engineering surfaces when viewed in a microscope is measured with the system, using image correlation. In this paper the equipment, physical model and calibration routines are described. The technique can be applied for sub-mm sized objects of arbitrary shape for small deformation fields. As a verifying experiment, an in-plane rotation of a flat calibration plate is presented. The expected in-plane errors are shown to be less than 0.1 μm and the corresponding out-of-plane errors about three times larger. As a pilot experiment, micro-structural paper expansion is studied, when exposed to humidity. The scaling properties of the microscope as well as the sampling criteria and reliability of the system are discussed in detail.

Compressive response and failure of braided textile composites: Part 1—experiments

Experimental results obtained by examining the planar biaxial compression/tension response of carbon 2D triaxial braided composites (2DTBC) are reported in this paper. These experiments were motivated by a need to examine the failure of 2DTBC in a state of stress that would be similar to what is experienced by the walls of a tubular member under compressive crush loads. Results obtained from a series of biaxial tests that were conducted with different proportional displacement loading ratio combinations of compression and tension are reported. In all cases, the dominant failure mechanism under such a stress state is the buckling of the bias and axial tows within the composite. Full field surface displacement data is acquired concurrently during all biaxial and some uniaxial tests using the technique of digital speckle photography. Digital images of the specimen surface that is illuminated with a He–Ne laser are acquired at discrete time intervals during the loading history using a high-resolution digital camera. These images are stored and analyzed to obtain the incremental inplane surface displacement field, Δ u( x, y) and Δ v( x, y). From these, the incremental inplane surface strains Δ ε x , Δ ε y and Δ γ xy are obtained by numerical differentiation. The present paper, which is the first in a two part series, is devoted to the biaxial experimental results pertaining to 2DTBC failure.

Failure Mechanisms and Modelling of Impact Damage in Sandwich Beams - A 2D Approach: Part I - Experimental Investigation

This paper addresses the effect of low velocity impact damage on post-impact failure mechanisms and structural integrity of foam core sandwich beams subjected to edgewise compression, shear and bending load cases. The study deals with a 2D configuration, where a sandwich beam is impacted by a steel cylinder across the whole width of the specimen. The impact damage is characterised as indentation of the core with sub-interface damage seen as a cavity while the GFRP faces remain virtually unaffected by the impact. Digital speckle photography (DSP) analysis is employed for in situ monitoring of crushing behaviour in the foam core during static indentation of sandwich specimens. The static shear strength of impact-damaged sandwich beams is compared with specimens with fabricated sub-interface cracks of the same length. DSP analysis reveals that the face-core interface in the peripheral regions of 2D impact damage is not entirely separated. The crack analogy is thus not fully representable since the surfaces remain bridged resulting in higher strength, when compared with fabricated cracks. The post-impact resistance to compressive loads is lower than for the specimens with fabricated cracks due to the presence of the cavity and the crushed core with reduced foundation stiffness support. The properties of crushed foam core are experimentally determined as they appear to be important for accurate modelling and analysis of the residual strength of sandwich beams. Modelling and post-impact analysis of the specimens with impact damage is elaborated in detail in part II of this study.

1114 デジタルスペックル写真法によるき裂先端応力場の解析

1114 デジタルスペックル写真法によるき裂先端応力場の解析

Digital speckle radiography—a new ballistic measurement technique

Digital speckle radiography is a measurement technique which is capable of visualizing internal flow fields within an opaque material undergoing a ballistic impact. This recent development has been successfully applied to the study of polyester, cement, sand and glass targets. The technique is based on digital speckle photography but, instead of producing the required speckle pattern by traditional means, the specimen is seeded with a layer of X-ray opaque filings. By utilizing two X-ray heads and varying the delays and position of the seeded layer, a full three-dimensional displacement map can be built up. The theories behind the technique are introduced, and results presented from two-dimensional displacement measurements on sand subjected to rod impact and three-dimensional measurements of cement being impacted by a shaped charge jet.

Experiments and analysis of laminates with artificial damage

Quasi-isotropic carbon/epoxy laminates with holes, polymer plugs and cut fibres are studied experimentally and analytically. Strain fields are measured using digital speckle photography. The results are used to validate an inverse method, where elastic properties of inclusions are determined by matching computed and measured displacements. Tensile and compressive strength is measured and the applicability of notch failure criteria to soft inclusions is examined. Available closed form solutions agree well with measured strains. The elastic properties predicted by the inverse method are in fairly good agreement with data from coupon tests although predictions are sensitive to measuring errors. The laminate toughness in compression was higher than in tension, as expected from the different failure mechanisms (fibre kinking versus fibre tearing). Laminates with inclusions were tougher than laminates with holes, which may indicate that inclusions restrain in-plane fibre kinking. Higher toughness was reflected in larger characteristic lengths.

Speckle tomography of turbulent flows with density fluctuations

The speckle tomography technique is used for reconstructing both large-scale structures in turbulent flows and the microstructure of turbulence. The technique is based on multi-projectional line-of-sight speckle photography measurements with a subsequent computer-assisted tomographic reconstruction of the interior structure of the flowfield. The large-scale structure is reconstructed using the Radon integral equation, and the microstructure is analysed using a statistical approach and a novel Erbeck–Merzkirch integral transform. Digital speckle photography and speckle tomography methods are described. Numerical simulation of the optical technique is performed using digital ray tracing through a turbulent flowfield. The methods are illustrated by the 3D "averaged" temperature fields in turbulent convective flows obtained earlier and by the recent reconstruction of 3D correlation functions of density variations in turbulent flows. Local values of turbulence (Kolmogorov) microscale are evaluated using these correlation functions and the Erbeck–Merzkirch integral transform The precision of the reconstruction and the spatial resolution achieved are analysed.

Out-of-plane displacement measurement by electronic speckle pattern interferometry in presence of large in-plane displacement

We measured out-of-plane displacement in presence of large in-plane displacements and deformations by electronic speckle pattern interferometry (ESPI). By means of digital speckle photography (DSP) the large in-plane displacement is measured and then compensated by software from interferometric images, before calculating the phase distribution related to the out-of-plane deformation. This means that decorrelation effects are not present and that the use of intermediate images is not necessary. The optical phase was extracted by spatial phase shifting, which enables the study of rapid transient events. The proposed method was applied to different combinations of out-of-plane deformation and large in-plane displacement. The compensation of large in-plane displacement allows us to maintain the contrast of ESPI fringes.

Digital speckle photography: visualization of mesoflow through clustered fiber networks.

Digital speckle photography (DSP) is used for velocity field measurements inside a fiber network. The width of the channels in which the flow is measured is typically less than 1 mm. Therefore a microscope is used to image the fiber network. When we sample 30 images/s and separate the moving parts of the images from the stationary parts, the velocity field can be deduced with DSP.

Optical in-plane strain field sensor.

A whole-field speckle strain sensor is presented. The speckle strain sensor allows the measurement of all three in-plane components of the strain field simultaneously without touching the surface of the sample. The strain fields are extracted from the in-plane motion of defocused laser speckles in a telecentric imaging system. To distinguish the contribution to the speckle motion from surface translation, rotation, and strain, the speckle motion from three lasers with different illumination directions and wavelengths has to be analyzed separately. Simultaneous acquisition of the three individual speckle patterns is achieved by means of splitting the light from the lasers onto separate but synchronized detectors with the aid of dichroic mirrors. The motion of the speckles is calculated with digital speckle photography (speckle correlation), which enables the strain sensor to measure strain fields with noise levels as low as 10 microstrain.

Hybrid experimental-numerical concept of residual stress analysis in laser weldments

The concept, methodology and instrumentation for hybrid experimental-numerical residual stress analysis in a laser weldment are presented. Grating interferometry and digital speckle photography are applied as complementary experimental methods for the determination of the initial model of residual strains and of the material properties at the various zones of a laser weldment. These data inserted into a finite element model enable one to analyze the formation of the residual stress state of the object, which is compared and modified by means of experimental data in a closed iterative loop. This full hybrid approach is tested successfully on a laser-welded steel specimen in uniaxial tensile tests.

White-light digital speckle photography in free convection

A digital white-light speckle system is proposed for measurements in free convection. White-light speckle photography has certain advantages with respect to traditional speckle photography. Theory of the method and its application to free convective flows in liquids are presented.

Speckle interferometry: three-dimensional deformation field measurement with a single interferogram

An electronic speckle interferometer, arranged for out-of-plane sensitivity and with an off-axis reference beam to produce spatial phase bias, is used for three-dimensional deformation field measurements. The complex amplitude of the object wave is calculated by application of a Fourier-transform method to a single interferogram. The change in phase after object deformation yields the out-of-plane component of the displacement field. The two in-plane components are obtained by cross correlation of subimages of the reconstructed object wave's intensity, a method that is also referred to as digital speckle photography. The Fourier-transform algorithm is extended and modified, leading to random measurement errors that are below widely accepted theoretical limits and also to an extended measuring range. These properties and the mutually combined information improve the accuracy of both methods compared with their usual single implementation. The performance is evaluated in experiments with pure out-of-plane, pure in-plane, and combined deformations and compared with theoretical values. An example of a practical application is given.

Pulsed TV holography combined with digital speckle photography restores lost interference phase

The measuring of situations with optical measuring methods is difficult when a deformation field must be determined while it is superposed to comparatively large rotating or translating object motion. Interferometric methods such as pulsed TV holography might be suitable to measure the small transient deformation, but the often-large bulk motion makes the phase information disappear. However, by a combination of digital speckle photography (DSP) (also called digital image correlation) with pulsed TV holography, such measuring problems can be mastered. A method to calculate the bulk in-plane motion by DSP from the usual pulsed TV holography recordings and then to use this information to restore the interference phase is proposed. This technique may be attractive in the study of transient vibrations overlaid on rotating or translating motions.

Experimental Determination of Stiffness Variation Across Growth Rings in Picea abies

Summary The radial and tangential deformations in a single growth ring of Norway spruce, which was subjected to moderate tensile loading in its radial direction, were measured by use of digital speckle photography. From the measured displacement field, the transverse coefficients of elasticity ER , ET , GRT and νTR were determined with respect to radial position in the growth ring. As a result of the abrupt change in stiffness from latewood to earlywood in the growth ring, it was possible to establish all four coefficients solely from the radial tension test. The elastic modulus ET was found to be a factor 15 larger in latewood than in earlywood, whereas the corresponding variation for ER was merely a factor 3. The measured variations were in reasonable agreement with predictions from simple two-dimensional honeycomb models of the growth ring cellular structure. The average elastic moduli of the growth ring were found to be twice as large as ER and ET measured in conventional tests on macroscale specimens, whereas the reverse was true for the shear modulus. An explanation to these differences is sought by analysing the loading conditions and deformation behaviour of macroscale specimens used in transverse stiffness testing of wood.